A frequency converter is a device used for instance for controlling a motor or another load. The control of a motor may be implemented reliably by means of a frequency converter for instance in such a manner that the motor accurately implements the desired speed or torsional moment instruction, for example.
One or more choke devices are typically associated with frequency converters. Examples of such choke devices possibly used in frequency converters or in association therewith include an input choke and an output choke.
The input choke of a frequency converter is a filtering device connected between a feeding network and the rectifier (e.g. alternating current bridge) of the frequency converter and serving to decrease the distortion of the current taken from the network and to protect the components of the alternating current bridge of the frequency converter from interferences and voltage peaks coming from the direction of the feeding network. In addition, the input choke may be used to attenuate the electromagnetic radiation of the frequency converter. In the case of a plurality of diode bridges, each diode bridge typically has an input choke of its own.
A possible output choke in a frequency converter, in turn, is connected between the inverter of the frequency converter and the device (load) it is feeding. The output choke of the frequency converter preferably restricts the derivative of the output voltage of the converter, thus protecting the device the frequency converter is feeding. When the device fed is a motor, the output choke protects the windings of the motor against partial discharges and restricts the bearing currents caused in the motor by the common-mode voltage generated by the pulse-form three-phase output voltage of the converter. Depending on the structure of the inverter, the choke device serving as its output choke may comprise one or more individual choke windings per phase. For instance in high-current inverter assemblies, it is known to connect a plurality of switch components in parallel to achieve the current-carrying capacity required, whereby the inverter comprises a plurality of output branches per each phase, each of which may be provided with an output choke.
In prior art choke device solutions for a frequency converter, the windings of the choke device are placed in parallel in a plane like manner. The problem in such prior art solutions is that the flow of cooling medium, such as air, through the choke device is uncontrolled; the cooling medium flows faster where the resistance encountered thereby is smallest. This being so, the cooling of one or some windings of a choke device comprising for instance three parallel windings may remain insufficient or at least require a disproportionately high flow of cooling medium. Furthermore, the heat stresses experienced by the different windings are unevenly distributed.